New high resolution neural interfaces allow for accurate spatial steering of therapeutic stimulation towards target tissue. Such high-resolution interfaces usually comprise an array-like distribution of elements (e.g. contacts or electrodes) capable of delivering stimuli (e.g. electrical pulses) and the array is usually placed on a carrier structure (e.g. an elongated flexible probe). FIG. 1B presents an example of such a high resolution neural interface used for the accurate delivery of electrical stimulation for the purpose of deep brain stimulation therapy. For comparison is provided in FIG. 1A on the same scale a conventional low-resolution neural interface for deep brain stimulation therapy. The electrodes shown in FIGS. 1A and 1B are also referred to as state of the art DBS lead and high resolution DBS array, respectively. It has to be noted that such a generation of a field in a tissue may also be useful in other applications such as ablation of tissue or even non-therapeutic applications.
The electrical potential in the brain tissue surrounding the DBS leads can be computed using the finite element method (FEM) as disclosed in Edsberg L., Introduction to Computation and Modeling for Differential Equations, J. Wiley, Wiley-Interscience: pp. 140-146, 2008, ISBN-13 9780470270851.
The electric potential V generated by the DBS electrodes is obtained by solving Poisson's equation as is disclosed in Bronzino J., Biomedical Engineering Handbook 2006, vol. I, section III, chapter 20, pp. 1-3, CRC. 1, ISBN-13 9780849304613:
                                                        ∇              2                        ⁢            V                    =                                    1              σ                        ⁢                          ∇                              ·                                                      J                    →                                    i                                                                    ,                            (        1        )            where ∇2 is the Laplace operator, {right arrow over (J)}i is the current source and σ the electrical conductivity.
For estimation of DBS activation volumes one can compute the activating function (AF). In general, the AF quantifies the driving force for depolarization of neuronal elements as is disclosed in Rattay F., The basic mechanism for the electrical stimulation of the nervous system, Neuroscience Vol. 89, No. 2, pp. 335-346, 1999.
As disclosed in McIntyre C. C., S. Mori, et al., Electric field and stimulating influence generated by deep brain stimulation of the subthalamic nucleus, Clinical Neurophysiology Volume 115 Issue 3, pp. 589-595, March 2004, estimation of the stimulation volumes is realized by thresholding the activation function distributions, which has been shown to provide a good initial estimate of the volume of activated tissue which is computed with more extensive computational modeling. The activation function is obtained by taking the discretized second spatial derivative of the external potential. For example for elements oriented in the z-direction, the activation function is computed as:AFz(x,y,z)=V(x,y,z−Δz)+V(x,y,z+Δz)−2V(x,y,z),  (2)
wherein the step length Δz=0.5 mm is the typical inter-node length for myelinated fibers. Activation occurs when the activation function crosses a certain threshold that is i.e. dependent on stimulation parameters (most notably pulse duration), fiber properties and relative fiber-electrode orientation.
In clinical practice, repositioning of stimulation fields will be considered when for a given stimulation configuration adverse side-effects are obtained. By steering the stimulation fields away from areas responsible for such side-effects one will try to avoid side-effects and simultaneously keep good therapeutic effects. In current clinical practice the usual method for displacing stimulation fields is by selecting a different contact for stimulus delivery. As this immediately displaces stimulation fields by 2-3 mm this is a quite coarse method. More fine control of stimulation field displacement can be achieved by current steering techniques and/or higher-resolution stimulation arrays.
Current steering is known in the art as a method to displace stimulation fields. In brief, the method consists of balancing current delivery between two or more contacts. For example, Butson, C. R. and McIntyre C. C., Current steering to control the volume of tissue activated during deep brain stimulation, Brain Stimulation 1(1): pp. 7-15, 2008, demonstrate how current steering can be used with a state-of-the-art DBS lead to tune the stimulation volumes, see FIG. 2. In this example, a total stimulation current is distributed over two adjacent electrodes and depending on the balance of current between the two electrodes a different activation profile results. FIG. 2 shows from left to right: a DBS electrode, a series of field contours starting from activation of a single electrode and ending with the activation of two neighboring electrodes of the DBS electrode. As is clear from FIG. 2, balancing the current between two contacts allows shifting the activation volume.
US 2007/0203539 discloses current steering with the high resolution DBS-array which is shown in FIG. 1B.